You don't need to even go as far as immortal. Just assume a human lived to 500 years on average. They would accumulate a lot of damage over that time (muscle t

You don't need to even go as far as immortal. Just assume a human lived to 500 years on average. They would accumulate a lot of damage over that time (muscle tears, concussions, broken bones that don't fuse properly, etc.). This would lead to a lot of people in your tribe needing help doing simple tasks. Which would cost your tribe energy. Instead reproduction takes some more energy in the beginning (feeding and teaching), but there's a break-even point where the offspring can provide for the next and previous generation more efficiently. I'm wondering if people would be less stubborn and more flexible if they died later. Otherwise you'd have people asking you to get off their lawn for 430 out of their 500 year lifespan.